1. Field of Invention
This invention relates to Hall thrusters that are used in propulsion systems. Specifically, this invention relates to systems and methods that allow for the improvements to Hall thrusters.
2. Description of Related Art
Hall effect plasma accelerators have received substantial scrutiny by the engineering community due to their unique capability for efficiently producing high energy plasma beams that can be used for space propulsion or for terrestrial material processing applications. Hall effect plasma accelerators, or Hall thrusters, as they are commonly referred, rely on an annular ceramic discharge channel in which plasma is ionized and accelerated. The plasma is accelerated by the combined operation of electric and magnetic fields applied in the coaxial channel.
More specifically, Hall effect plasma accelerators rely on a magnetic field established across an annular dielectric discharge chamber and a working fluid, typically gaseous xenon, which is introduced at the rear of the annular discharge chamber through an anode-gas distributor. A plasma discharge is established by applying a voltage between the anode-gas distributor and an external cathode. The magnetic field is used to impede the flow of electrons from an external cathode to the anode allowing electric field strengths sufficient to produce high ion energies (typically 200-1000 Volts). Hall effect plasma accelerators provide high jet velocities, in the range of 10 km/s to 20 km/s, with current densities, about 0.1 A/cm2. The input power levels for most thrusters are in the general range of 0.5 kW to 10 kW.
While most Hall thrusters retain the same basic design, the specific details vary with the nominal operating parameters, such as the working gas, the gas flow rate and the discharge voltage. The general design parameters that are varied to meet specific requirements include the discharge channel geometry, the material for that channel, and the magnetic field distribution. The discharge channel is typically made of boron nitride, but other compositions are possible.
One or more magnetic sources, in a Hall effect plasma accelerator, in a particular arrangement form a magnetic circuit. In prior art Hall effect plasma accelerators, magnetic fields are produced that are substantially radial. These magnetic fields allow for the erosion of the dielectric discharge chamber by the high energy ions contained within it. Ultimately, this results in erosion of the surrounding magnetic system.
The operational lifetime of the accelerator is defined by the amount of time the accelerator can operate before the magnetic system is exposed to the plasma within the channel. The lifetime of state-of-the-art accelerators is on the order of 10,000 hours. Thus, if there is a means of ensuring that the magnetic system is not exposed by erosion of the ceramic discharge channel, then the useful lifetime of an accelerator can be extended.
Several methods have been employed in the prior art to increase Hall thruster lifetime. Attempts have been made to identify and incorporate discharge chamber materials with high resistance to erosion. Prior techniques for extending operational lifetime include increasing the thickness of the discharge channel material, magnetically shielding the discharge channel material from the plasma, and controlling the energy of the plasma interacting with the discharge channel.
However, none of the prior techniques implemented have eliminated the life limiting mechanism of Hall thrusters. Additionally, some of the prior techniques introduced negative effects on thruster performance. Thus, there is a need in the prior art to have Hall thrusters with increased usable lifetimes.